1. Field of the Invention
The present invention relates to a lens barrel capable of focusing by an ultrasonic wave motor incorporated therein, and more particularly to a lens barrel capable of automatic focusing or powered focusing by the driving force of an ultrasonic wave motor and also of manual focusing by a manual operation ring.
2. Related Background Art
There are already known different mechanisms for effecting automatic focusing and powered focusing with an ultrasonic wave motor, and methods for switching between an automatic mode by an ultrasonic wave motor and a manual mode by a manual operation are disclosed for example in the Japanese Laid-open Patents No. 59-101608 and No. 61-86718. Also the control of the ultrasonic wave motor is disclosed in the Japanese Laid-open Patent No. 61-65214. Furthermore, a condition in which the ultrasonic wave motor is stopped by the destruction of a resonance state when the pressure between the stator and the rotor exceeds a nominal value is reported in the Journal of Society for Electronic Information Communication, Vol. 70, No. 7, pp 717-720, July 1987.
In the above-mentioned prior art, there is a structure in which the rotor of the ultrasonic wave motor is directly connected with or disconnected from a driving member for a focusing lens group by means of a mechanical clutch member. However, the experiments of the present inventors have revealed that the clutch member requires a considerable axial pressure to the rotor, in order to securely couple the clutch member to the rotor, particularly in a rotating state. Consequently there is a drawback in that the pressure between the rotor and the stator exceeds the above-mentioned nominal value, thus destroying the resonance state and eventually stopping the motor.